1. Field of the Invention
This invention relates to a package for an integrated circuit device and a method of making same. More particularly, this invention relates to a chip on board multilayer printed circuit board package for an integrated circuit device and a method for making the package utilizing a self-aligning photoexposure process.
2. Description of the Prior Art
The need for an increased number of leads in the packaging of Very Large Scale Integration (VLSI) devices has resulted in the development of chip carriers which utilize terminals on four sides of the package instead of the conventional two sided Dual In-Line packaging. It has also resulted in the development of pin grid arrays in which a plurality of rows of pins are arranged in an array, for example, with orthogonal spacing. This latter type of packaging is preferred when the pin density approaches 100 or more.
Pin grid array packages usually utilize multilayer ceramics or printed circuit (PC) boards. The costs associated with the use of multilayer ceramics as well as the superior conductivity of the copper traces used in PC boards favor the use of PC board technology for such packages. PC board packaging technology has long been used in watches and video game modules. In such applications, the integrated circuit die is directly attached onto a polymer-impregnated fiberglass board (PC board) with a conductive bonding material, e.g., a silver-filled epoxy resin, wirebonded out to the copper circuit traces on the PC board, and then encapsulated, for example, with an epoxy or silicone resin. Mounting pins to the board, either by a pressfit or by soldering in place, creates a simple PC board type pin Grid Array package.
However, many VLSl devices generate considerable heat, necessitating the use of heat dissipation devices such as heat sinks. In PC technology, this can involve cutting out a central hole in the board, mounting a copper slug in the hole, and then bonding the chip directly to the slug. Such a package, when utilized in a cavity-down configuration where the heat can transfer more easily to the air flowing over the package, can dissipate heat more efficiently than an equivalent ceramic package. However, while the use of a copper slug in a PC board package does dissipate heat well, the thermal mismatch between the copper and the silicon substrate of the integrated circuit die bonded thereto can cause damage to the die.
An alternative form of heat dissipation utilizes heat tubes comprising holes in the PC board which are plated to provide a heat path between the metallization on one side of the PC board to which the integrated circuit die is attached and a metallized pad on the opposite side of the PC board. This type of structure is less costly and more reliable than the use of a copper slug.
However, such a heat tube arrangement can create a moisture path to the die if the hole is not filled up prior to encapsulation. Therefore, heat tube arrangements are more advantageously used in multilayer laminates where the die attach metallization extends between the layers or laminates and the plated holes extend through only one laminate which is then bonded to the other laminate to which the die is mounted with the metallization between the laminates thus creating blind holes. While such an arrangement solves the problem of possible moisture conduction to the die through the heat tubes, it complicates the manufacturing process by requiring two separate plating steps; the first to plate the heat tubes prior to laminating, and the other to plate the holes for the pins. Plating the holes for the pins must be done after the lamination step since the holes for the pins are usually not drilled until after lamination to avoid problems of misregistry which could occur if the holes in each layer or laminate were drilled separately prior to lamination.
Another problem encountered by the prior art relates to outgassing from the ragged edges or laminate adhesive exposed when the cavity to mount the die is punched out. Such outgassing can interfere with subsequent encapsulation of the package. This exposed area could be effectively sealed off by plating. However, the use of electroless copper plating techniques require the surface of the PC board to be very accurately masked off to prevent plating of the surface which could, in turn, short out all the wire traces previously patterned on the surface. One prior art approach to this was to provide a photoresist mask over the entire surface prior to punching out or routing the die cavity.
This, of course, provides the desired mask right up to the edge of the die and thereby permits plating (and sealing) of the sidewalls exposed by the punching step.
This solution to the problem of outgassing, however, rules out the use of heat tubes in a multilayer laminate structure since the heat of lamination could damage the photoresist mask which could, in turn, have a deleterious effect on the yield due to the need for a second plating step after lamination of the layers to plate the holes for the pins as discussed earlier.
Thus, in the prior art constructions of PC board packaging, one had to either live with the problem of outgassing--by not plating and sealing the punched out cavity walls of the laminate--or with the problem of thermal mismatch between the copper slug heat sink and the integrated circuit die if one did plate the cavity walls since the heat tube approach could then not be utilized.
It would, therefore, be very desirable if one could construct a PC board package for integrated circuit dies having plated cavity walls to prevent outgassing and yet utilize a multilayer heat tube type heat dissipation system.